The present invention relates to devices and methods for zonal isolation of well bores. More particularly, the present invention relates to zonal isolation devices and methods of use for performing multiple stage downhole stimulation operations.
Downhole production stimulation operations include operations such as hydraulic fracturing operations and acid stimulation operations. Hydraulic fracturing operations generally involve pumping a treatment fluid (e.g., a fracturing fluid) into a well bore that penetrates a subterranean formation at a sufficient hydraulic pressure to create or enhance one or more cracks, or “fractures,” in the subterranean formation. Once at least one fracture is created and the proppant particulates are substantially in place, the fracturing fluid may be “broken” (i.e., the viscosity of the fluid is reduced), and the fracturing fluid may be recovered from the formation. Other production stimulation operations include acidizing treatments in which an acid is introduced into the subterranean formation to create or enhance channels or pores in the subterranean formation so as to increase the permeability of the formation.
In typical stimulation operations of subterranean formations, stimulation treatments may be independently performed in multiple stages by introducing stimulation treatments separately as to different zones along a well bore or well bores. These multiple stage treatments may be performed simultaneously, but often, it is advantageous to perform the multiple stage stimulation treatments independently and/or sequentially. Often, it is desirable to individually isolate each portion of the subterranean formation to be treated so that a stimulation treatment fluid may be introduced into a desired portion of the subterranean formation. In such multiple fracturing treatments, zonal isolation may be necessary, at least temporarily, to direct or bias the stimulation fluid into a desired portion of the subterranean formation. As used herein, the term “zone” simply refers to an area or region and does not imply a particular geological strata or composition.
Conventional methods for isolating zones or portions of subterranean zones include methods such as the ball and baffle method. In this conventional method, a series of baffles may be placed in the casing string, with each baffle being placed at a point in the string corresponding to the base of a zone or interval to be perforated and stimulated. The baffles may be arranged in order of decreasing inner diameter, with the smallest inner diameter baffle located at the base of the second lowermost zone to be stimulated. In this way, after the casing string is cemented in the well bore, the lowermost zone may be perforated to allow a stimulation treatment to be applied to the lowermost isolated zone. After completion of the stimulation treatment of the lowermost stage, the stimulation fluid may be recovered and the zone above the lowermost baffle may be perforated in preparation for a later stimulation operation. Then, a weighted ball may be introduced into the casing string that is sized to seat on the lowermost baffle. Because the baffles are usually arranged in order of decreasing inner diameter, the weighted ball may pass through all of the upper baffles, finally seating on the lowermost baffle. That is, the weighted ball may be small enough to pass through all of the upper baffles having larger inner diameters, yet be large enough to seat on the lowermost baffle, providing fluid isolation beyond the lowermost baffle. Then, once the zone below the ball and baffle is isolated from fluid communication with the zone above the ball and baffle, the zone above the ball and baffle may then be stimulated. The zonal isolation between the two zones allows the zone above the baffle to be stimulated while not being affected by possible fluid loss to the first stimulated zone. After this second stimulation treatment, the stimulation fluid may be recovered along with the weighted ball.
Subsequently, the next higher zone of interest may perforated to allow treatment of the next stage or zone with a stimulation treatment, such as a fracturing fluid or an acidizing treatment fluid. Another weighted ball sized to fit the next larger baffle may be introduced into the casing string to provide zonal isolation of the next higher zone of interest. Similarly, subsequent zones may be treated in a like manner until all zones isolated by the baffles are stimulated, after which the baffles may be drilled up if desired and the well cleaned up in preparation for production.
Conventional ball and baffle methods are often used in wells that are generally vertical, relatively cool (e.g., less than about 200° F.), and where the hydraulic pressure required for the various stages of fracturing is generally less than about 4,000 psi. Unfortunately, the ball and baffle method is limited to casing strings comprised of API threaded and coupled casings. Moreover, such methods may be difficult to carry out in wells that are either deviated wells, high temperature wells, or wells in which the fracturing pressure require high pressures. One reason that such methods may be unsuitable for deviated well bores is because the ball and baffle method relies on the free-falling of the weighted ball through the series of baffles, and the weighted ball may experience difficulty in passing through one or more of the baffles because of the non-vertical trajectory associated with a deviated well bore. Additionally, because the material of the weighted ball is often made of a drillable material, weighted balls are generally not capable of withstanding the high temperatures and pressure of certain wells without physically deforming. Further, another common disadvantage of ball and baffle methods is that the recovery of the weighted balls relies on the ability of the flow of the recovered stimulation fluid to carry the weighted ball back out of the well bore during recovery of the stimulation fluid. In some systems, the flow rate of the fluid being recovered is not sufficient to return the ball to the surface, which results in the necessity of drilling the weighted ball out of the casing string, which is undesirable because it adds additional undesirable complexity, cost, and time to the downhole operations.
Another conventional method of providing zonal isolation involves the use “frac plugs,” which are sometimes referred to as “bridge plugs.” In this method, frac plugs may be set at the base of each zone to be stimulated. This method, however, may require an undesirable amount of time and expense, because each frac plug has to be run and set with an individual trip into the well bore with an electric line or tubing. In situations where a drilling operator drills a number of multiple wells successively, the additional trip time for the placement of each frac plug can become quite onerous and expensive. Additionally, after completing the stimulation of all of each zone, each of the frac plugs must be drilled up to put all of the zones in production. Furthermore, the time required to complete all zones using this conventional method may be excessive, in some instances taking up to several days to complete.